Determining the position of a cylinder piston represents a necessary and important task in a series of technical applications. In particular, the exact position of the cylinder piston often plays a major role in targeted control of the piston-cylinder unit. Moreover, determining the position can decisively increase the operational reliability of a piston-cylinder unit, since the intake of the hydraulic medium, in particular the hydraulic fluid, at the extreme positions of the cylinder piston can be exactly controlled and consequently stopped at the right time.
Precise position determination is also important in the automatic control of the piston-cylinder units in construction machines and hoisting equipment. The piston-cylinder unit actuates the working devices of the construction machine or hoisting equipment in the usual manner. Sufficiently precise determination of the position of the piston-cylinder unit increases the quality of the control, and therefore is absolutely essential.
The high control pressures prevailing within the piston-cylinder unit, which occur especially in hydraulic cylinder units, often allow only slight modification of the piston or the cylinder jacket without interfering with the entire system in a way that is relevant to safety. For this reason, setting up a suitable position measuring device often turns out to be especially difficult and cost-intensive.
Numerous piston-cylinder units pick up the instantaneous position of the cylinder piston by cable potentiometers.
Processes are also known that work according to a magnetostrictive principle. These involve picking up the position of the piston using a ring magnet attached at a specific piston position in combination with a sensor built into the piston rod.
However, one thing that all known processes have in common is that they require elaborate and cost-intensive modifications of the piston-cylinder unit. Additional transducers and sensors must first be integrated into the piston-cylinder unit.
The present disclosure has the goal of pointing out a piston-cylinder unit with a device for determining position that has satisfactory stability and robustness but is nevertheless simple and economical to produce and attach.
This is accomplished by a piston-cylinder unit with a device for determining position. It is possible for the piston-cylinder unit to be made in the form of a hydraulic cylinder and for it to use a hydraulic oil as its hydraulic medium.
The device for determining position comprises at least one exciter that is electrically connected, indirectly or directly, with the cylinder jacket and with the cylinder piston of the piston-cylinder unit. In this arrangement, the cylinder jacket and cylinder piston function as the electrodes of a series-resonant circuit. The piston rod and cylinder jacket form a series inductance. Opposite surfaces of the piston and cylinder jacket form a capacitance with hydraulic media. Accordingly, the complete piston-cylinder unit can be understood as an oscillating circuit.
The exciter according to the present disclosure serves to excite the electrical series-resonant circuit to oscillate at its resonant frequency. In theory, the resonant frequency that is set up in an oscillating circuit is a function of the capacitance and inductance. Consequently, it is possible to deduce the variable capacitance of the piston-cylinder unit from the resonant frequency, the capacitance and inductance depending on the instantaneous position of the piston. Thus, it is possible to determine the exact instantaneous position of the piston from the resonant frequency. To accomplish this, it is also possible to measure an electrical signal characterizing the resonant frequency on the device according to the present disclosure.
The present disclosure utilizes the advantage that the piston-cylinder unit is suitable, without modification, to form an electrical oscillating circuit. In contrast to the prior art, it is not necessary to arrange any external sensors or transducers or additional electrodes on or inside the piston-cylinder unit. The known components of a piston-cylinder unit, such as the cylinder jacket and the cylinder piston, are used to form a series-resonant circuit. Thus, in one example, the piston-cylinder is without an external sensor or transducer or additional electrodes on or inside the piston-cylinder unit for position detection.
The exciter advantageously comprises an oscillating circuit that is electrically connected with the piston-cylinder unit. A Hartley oscillating circuit is especially advantageous.
The resonant frequency of the oscillating circuit is a high-frequency signal, known from experience to lie in the megahertz frequency band. The piston-cylinder unit can act as an antenna that emits electromagnetic waves. In this connection, it can be expedient for at least part of the device for determining position to be advantageously arranged inside of the cylinder housing or cylinder jacket. In particular, the exciter is arranged inside the piston-cylinder unit or a hollow space in the piston-cylinder unit provided for this purpose. The shielding effect of the cylinder jacket has an advantageous effect on the EMC characteristics [electromagnetic compatibility] of the device or the piston-cylinder unit.
Alternatively, it is possible that at least one additional shield to be provided that covers the externally arranged device for determining position, in particular the exciter, and prevents the emission of electromagnetic waves. It turns out to be advantageous for the shield to be magnetic, in particular made from a ferromagnetic material. Of course other shielding materials are also conceivable that are suitable to cover and shield the device for determining position. It also turns out to be advantageous to use at least one EMI filter [electromagnetic interference filter].
It is possible for the measured signal characterizing the resonant frequency to be an electrical voltage. This voltage has an oscillating signal shape during the oscillation of the piston-cylinder unit; it is advantageously electrically insulated, and is used as a square wave signal for digital evaluation.
Under some circumstances, the oscillating circuit components or external influences are damped and interfere with the oscillation behavior. To keep the oscillation amplitude constant, it can be advantageous for there to be a circuit device to stabilize the measurable voltage. External influences include, for example, moisture, dust deposits, etc. This measure stabilizes the resonant frequency and makes it possible to determine the position with sufficient accuracy.
It is possible to provide an evaluation device that is suitable to evaluate the signal and output the current piston position. The evaluation device can be an appropriately configured microcontroller or a suitable analog circuit device having computer readable storage media and code therein to carry out the various actions described herein. The evaluation device is either solidly or detachably connected with the piston-cylinder unit.
To reduce the resonant frequency, it can be advantageous to arrange an additional inductance between the exciter and the piston or between the exciter and the cylinder jacket. This can be advantageous for technical reasons involving EMC.
It is advantageous for the contact between the exciter and the piston-cylinder unit to be a sliding contact. The contact between the moving part of the piston-cylinder unit, in particular the piston rod, may be made by a sliding contact. It has turned out to be expedient for the contact between the piston rod and the exciter to be a brush type of contact, the brush sliding along the surface of the piston rod as the piston moves. it is possible for the brush to consist of carbon, bronze, or another suitable material.
Alternatively, the contact between the piston rod and the exciter can be made by a capacitive or conducting ring. The ring is arranged so that it can slide coaxially on the surface of the piston rod. Using a capacitive ring creates an additional constant capacitance, which is connected in series with the oscillating circuit.
It is possible for the ring to consist of a conductive material that is indirectly or directly connected with the exciter, a dielectric being arranged between the ring and the piston rod, or the conductive ring being directly electrically connected with the piston rod. It is also possible to isolate the piston-cylinder unit from the oscillator or the exciter by inserting a transformer. The leakage inductance of the transformer can also be used to reduce the resonant frequency.
Moreover, the present disclosure relates to a construction machine or a piece of hoisting equipment with a piston-cylinder unit described in one of the preceding advantageous embodiments. The construction machine or a piece of hoisting equipment according to the present disclosure has the same advantages and properties as the piston-cylinder unit described above, for which reason it is not explained again here.
The use of the piston-cylinder unit is not in any way limited to construction machines or hoisting equipment. Possible areas of application are found in aircraft or generally in all machines/equipment with hydraulic/pneumatic technology.
Further advantages and details of the present disclosure will be explained detail below using the sample embodiments shown in the drawings.